Ever wondered what went through author JK Rowling’s mind when she wrote the first Harry Potter novel?

Fans of books about the boy wizard will have a chance to bid on a unique first edition of Harry Potter and the Philosopher’s Stone, annotated by the author, at a charity auction this month, the English PEN organisation said.

The book contained Rowling’s handwritten thoughts and commentary about the book and the film adaptation, as well as 22 hand-drawn illustrations including one of a sleeping baby Harry and another of Hogwarts headmaster Albus Dumbledore.

It includes a 43-page ‘‘second thoughts’’ segment from the author, with phrases such as: ‘‘I wrote the book... in snatched hours, in clattering cafes or in the dead of night. For me, the story of how I wrote Harry Potter and the Philosopher’s Stone is written invisibly on every page, legible only to me...’’

The book will be one of 51 first editions annotated by authors at an auction on May 21 that will include Bridget Jones’s Diary by Helen Fielding, Northern Lights by Philip Pullman and Roald Dahl’s Matilda with original illustrations by Quentin Blake.

The Harry Potter fantasy series of seven novels, based on the adventures of a boy wizard, has become the best-selling book series ever and has been adapted into a multibillion-dollar film franchise.

Harry Potter and the Philosopher’s Stone was first published in 1997 and is the rarest of the series as only 500 copies were printed.

In an October 2007 auction, a first edition of the book sold for £19,700 (NZ$38,000) and the auction at Sotheby’s in London is likely to fetch high prices for the unique copies.